In recent years, the thin film device constituting a liquid crystal display panel or an EL display panel or the like used for a portable telephone or a portable information terminal such as a PDA has been increasingly requested to be rugged, thin and light. For example, in the liquid crystal display panel, in order to meet these demands, there has been partly used a technology that a counter substrate is bonded to a driving substrate and then the glass substrate is thinned by a mechanical polishing or a chemical etching process as well as a process that the glass substrate is initially thinned.
However, since the thin film device is manufactured under an environment of vacuum and high temperature, a substrate used for manufacture has a limitation. For instance, in a liquid crystal display device using the thin film transistor (TFT), a quartz substrate capable of enduring the temperature of 1000° C. and a glass substrate capable of enduring the temperature of 500° C. are employed. Although a method for thinning these substrates is studied, as long as the quartz substrate and the glass substrate are used, the size of the substrate needs to be reduced to prevent the rigidity of the substrate from being deteriorated, a productivity is lowered. Further, since the processing time of a glass thinning process is increased, the practical thickness of a driving substrate forming a display panel cannot be inconveniently reduced to 0.3 to 0.4 mm or smaller. Further, when the thickness of the glass substrate is reduced, a deflection is increased inversely proportionally to the cube of the thickness of the glass substrate. Thus, when the glass substrate is applied to a portable information terminal, an important ruggedness is abruptly deteriorated, which results in a factor for restriction not lower than the productivity. As described above, a performance required for a produced substrate is different from a performance required when it is actually used.
Therefore, it has been requested to use a thin and light plastic substrate in place of the glass substrate serving as a base of the thin film transistor substrate employed for the liquid crystal display panel or the like, however, the plastic substrate may be used with high difficulty in view of heat resisting temperature.